The invention relates to a fuel injection system with fuel pressure intensification, in which there is arranged, at the transition from a pressure source, which is formed in particular by a pressure accumulator, to a fuel injection injector, a pressure intensifier having a control space whose pressure level determines the degree of intensification and, consequently, the pressure increase over the initial pressure in the pressure accumulator and the operating pressure for the injector. The nozzle needle is loaded in the closing direction by the fuel pressure in a pressure chamber formed at the rear side of the nozzle needle.
In a known system of this type, the control space and also the pressure chamber are connected to a fuel return independently of one another, in each case by way of a control valve so that, by control of the pressure intensifier, a particular shape for the pressure curve can be established. By controlling the injector independently a range of the pressure curve suitable for injection can be selected. A mutually independent control of the pressure intensifier and of the injector is provided by magnetically actuated 2/2-way valves. This requires in addition to space and costs, a highly accurate coordination in the activation of the injectors and the valves since even small tolerances result in marked differences in the injection behavior.
In order to reduce the space requirement and the costs, and also to simplify the control so as to be able to sufficiently affect the engine combustion, in a first solution according to the invention, the connection of the control space and of the actuation space to the fuel return is controlled in a combined manner via a common valve connection. As a result, although the breadth of variation is restricted, the expenses are considerably reduced and space is saved, still wide-ranging possibilities for exerting influence are afforded. That is the combustion behavior of the internal combustion engine can be sufficiently influenced particularly with regard to obtaining favorable exhaust gas values.
A further solution according of the invention utilizes the fuel injector itself as a control element or control valve in that a part, which is involved in actuating the nozzle needle and moveable together with the nozzle needle, is a spool valve, which, by the design of the control cross-section thereof, particular the rising flank of the pressure profile can be varied in relation to the pressure prevailing at the nozzle needle seat.
Finally, in a further solution according to the invention the connection of the control space or of the action space to the return is influenced by a control valve in the form of a pressure balance with connections which branch off, on one hand, from the inlet and, on the other hand, from a connection between the inlet and the connection of the actuator space or of the control space to the return. In this solution, a separate valve control is implemented for connecting the control space and the actuation space to the return, but the control outlay is substantially reduced depending on the hydraulic conditions. The other valve control, which is disposed in the connection of the actuation space or of the control space to the return and which is established via the directional valve, affords the possibility of influencing the hydraulic conditions and consequently the control behavior of the pressure balance by a corresponding timing of the directional valve.
In a preferred design, one of the connections is branched off from the inlet and the other is branched off from a throttled connection between the inlet and the connection of the actuation space to a downstream control valve.
In such a design, the injector and the pressure intensifier are activated virtually simultaneously. A rising pressure profile during injection is thereby ensured.
This, in turn, makes it possible to provide for a process sequence which makes it possible, in particular, to affect the rising ramp of the pressure profile at the nozzle seat and which leads to a virtually rectangular pressure profile, in particular in the region of the rising ramp.
For this process sequence, it is assumed that the closing position of the magnetically activated control valve corresponds to a closing position of the nozzle needle due to the high pressure prevailing in the actuation space and due to the blocking of the control space to the return as determined by the pressure balance. When the magnetically activated control valve, which is in particular a 2/2-way valve, is briefly opened, the pressure drops in the actuation space and, with some delay, also in the control space. As a result, the pressure on the control piston of the pressure balance in the direction of its closing position is reduced. In this intermediate phase, however, the nozzle needle is still closed, so that, in the event of a correspondingly brief opening of the control valve, the pressure in the actuation space is reduced, but not the pressure prevailing at the inflow side, assuming corresponding dimensioning of the throttle cross-sections in the inlet and in the outlet to the actuation space. If, then, the control valve is once again opened, initially the pressure in the action space is reduced along with the high pressure at the inlet side, so that, during the opening of the nozzle needle, a correspondingly steep pressure rise at the nozzle-needle seat, and consequently, a steep pressure ramp, is obtained. This occurs especially since the previous lowering of pressure as a result of the preceding brief opening of the control valve also causes a reduction of the pressure acting on the control spool of the pressure balance in a closing direction. Consequently, when the control valve is once again opened in order to initiate fuel injection, there is a rapid displacement of the control spool of the pressure balance to the opening position connecting the control space to the return line.
In a further design, in which the pressure balance is branched off, on the one hand, from the inflow and, on the other hand, from a throttled connection between the inflow and the connection of the control space to the following control valve, when injection is initiated by opening of the control valve, the pressure intensifier is cut in before the injector is released. The result of this is a high pressure prevailing at the injector when the latter responds, this, in turn, entailing a steep, virtually rectangular pressure profile during injection.
Overall, therefore, in the solutions according to the invention, with only one magnetically activated control valve, an operating behavior is achieved, in which tolerances in the operation of the valve are avoided and the space requirements and also the control requirements are reduced overall, and in which, irrespective of these simplifications, both, fuel preinjection and fuel post injection are possible and the injection pressure is freely selectable.